I think rcg is right. I just tried 2 different CS-1 batteries and when I put into the 8700g, I get a picture of a bttery with a slash through it..and then nothing happens. That sux. Looking at the 8700 battery (CS-2) it has a picture of a lock on it.

I use my girlfriend's 7130e's battery in my 8700 and vice versa frequently, but only as a last-resort-mine-is-dead option, I've never tried charging it. Pretty sure it's also a CS-2 however so it's a rather moot point ;)